4. MAGNESIA ALBA differs essentially in its chemical and medicinal properties from every other known absorbent earth; and when mixed with an acid, either before or after its admission into the stomach, is _purgative in a much smaller quantity_ than chalk, crab's eyes, or any of the calcareous or testaceous earths. 5. The calcination of Magnesia divests it of those disagreeable properties complained of by Hoffman, and other practitioners; the fixed air which constitutes so great a share of its composition, and is the cause of the uneasy sensation produced by this powder, being expelled in the process. But depriving the Magnesia of its air does not render it caustic or unfit for internal uses. 6. It is a common, but unchemical practice, to mix acid and alkaline substances in the same composition, without attending to the changes which will be produced in their nature by being united. Among other instances of this kind which might be pointed out, we often meet with lenitive electuary, cream of tartar and Magnesia Alba prescribed together in one medicine, the consequence of which is, that the Magnesia is not only unintentionally neutralized, but the effervescence produced in it by the acid occasions the electuary to swell and renders it unsightly, besides altering the nature of the pulps and syrup which enter the composition, by inducing a vinous fermentation in them. The calcined Magnesia being a non-effervescent may be united with acids under this form without any other inconvenience than their producing a neutral salt, and the propriety of this change must depend on the intention of the prescriber. 7. It appears that Magnesia Alba, though remarkably septic to animal flesh, retards the putrefaction of bile, and restores sweetness to it when actually putrid: That these last effects are still more strongly produced by the calcined Magnesia, which also powerfully resists the corruption of flesh: That some of the other absorbents prove antiseptic to bile; and consequently that the opinion of the universal septic property of the absorbent class of medicines, and of the impropriety of prescribing them in bilious diseases, may admit of some exceptions. 8. Magnesia, when calcined, has the same property as quick-lime of promoting the solution of resinous gums in water. 9. The increased power of water as a menstruum to vegetable astringents, depends on only such a quantity of lime being employed as can be saturated with air by the _solvend_; but if a larger proportion be used, the action of the water on the vegetable is rather prevented than promoted. 10. Though lime water in several instances appears to be a more powerful menstruum to vegetables than distilled water; yet the latter is sometimes preferable, and acts more efficaciously than when impregnated with lime.